Treating silicon steel



Patented Nov. 7, 1950 TREATING SILIGON STEEL 'Charles H. Campbell, Bedford, and Donald 'S. Ferito, Cleveland, Ohio, "and Carl W. Stoker, Vandergrift, Pa., assignors of one-half to The American Steel and Wire Company "of New Jersey and one-half to Carnegie-Illinois Steel *Gorporation, :both corporations of New Jersey -'NoDrawing. ApplicationMarch25,1947 Serial No. 737,086

2 Claims. (01. -I4812.5)

This invention relates to .a method for improving the .:magnetic characteristics of silicon l steels, particularly the :preferred oriented silicon steels :such -as are *used in making transformers and the like.

Efn the :m-anufactureof transformer grade silicon steel strip, it .is customary to anneal the strip to lower the carbon content after which the strip is again annealed to develop proper magnetic properties. -We have discovered that bylproperly controlling the scale developed on the strip-during the :decarburizing anneal followed by a second anneal with a propertseparating medium between the sheets or strip, that a pronounced improvement in magnetic properties is obtained.

:In annealing silicon strip in conventional atmospheres, .a layer of scale is formed thereon, underneath which there is formed .a layer of sub-scale. The outer layer tends .to be either .a dense .silica bearing scale which prevents .proper decarburizing, or almetalliclscale which causes badsurface conditions and adversely affects the magnetic properties. The sub-scale resulting from'heating silicon steel in an oxidizing atmosphere is a well-definedzone of low silicon iron interspersed with particles of silica or of iron silicates. Such sub-scale is more fully described in an article entitled Diffusion in Metal Accompanied by Phase Change, by L. S. Darken .and published as Technical Publication No. 1.4.7.9 by the American Institute of Mining and Metallurgical Engineers. The article appeared in the August 1942 issue of Metals Technology. We have discovered that by proper combination of atmosphere and temperature, a layer corresponding to the structure and composition of such sub-scale can beformed directly on the surface of silicon steel sheets without any superimposed layer of scale. It has been demonstrated that the combination-of a layer of scale corresponding to this sub-scale, together with box annealing with 'a lime separator coating is a decisive factor in achieving the desired improvement in magnetic properties.

The treatments herein disclosed may be applied to silicon steels generally, that is, electrical steels containing between .50 and 5% silicon, but is more particularly related to such steels containing between.2.5% and 3.5% .silicon and which are processed so as to develop preferred grain orientation. Regular grades :of silicon steel containbetween .06 and .-l2% manganese, the manganese being reduced to such low levels because oi its adver'se-afiect on obtaining the low carbon desired, but "with our treatment manganese contents of 25% or more are permissible.

Broadly the treatment consists in cold-reduw 2 ing hot-rolled strip to the desired gauge or at least about-.03 inch gaugeafter which it is ann'ealed in an atmosphere and at .a temperature which :willgive .thedesired scale. We have found that an atmosphere containing-about 18 to 23% hydrogen, balance substantially nitrogen, the gas being saturated with water vapor at room temperature develops .a satisfactory scale when the annealing temperature .is between 16Q0 and 1725' 'F. Following this annealing, the strip is coated with a lime separator and given a' high temperature anneal at a temperature between 1900 and '2-100 F., preferably at about 2000 in a nitrogenatmosphere to develop the desired magnetic characteristics.

To obtain proper magnetic properties, the cold reduction is carried out in two steps. The full processing procedure for such material is as ran lows: silicon steel strip is hot rolled to above .638 inch gauge, pickled in acid, Washed, dried, continuously cold reduced to an intermediate gauge, such as about .026 inch continuously annealed in the atmosphere controlled to develop the scale hereinbefore described, at .a tempera-- ture'betwen 1600 F. and 1725 F., using a rate of travel of about 5 feet per minute, corresponding to 5 minutes at temperature, cold reduced to the finalgauge desired such as ..014 inch, again continuously annealed in anatmosphere similar to the first with similar time and temperature .con-. .ditions. The strip is then dipped in a lime slurry, dried,.annea'led at 2000 F. in an atmosphere of nitrogen. Following this, the strip may be given a stress-relief anneal by heating it for one hour at 1425 F. in a closed-muffle furnace having an atmosphere formed by keeping the residual gases in the mufile.

As before stated, we havefound that an atmosphere suitable for developing the desired scale m'ay'con'tain 1-8 to 23% hydrogen, balance nitrogen, the gas being saturated with water vapor at room temperature. The gas may contain up to about 1% each of carbon monoxide and methane. The important point in connection with the gas is that it prevents the formation of a heavy silica hearing or metallic surface scale but per"- m'itsa sub-scaleto form directly on the surface whereby a well-defined zone of low silicon iron interspersed with particles of silica or iron silicates is built on the metal. Thus, this sub-scale i in effe'cta'layer of steel with a reduced silicon content. Also this layer is colorless as compared to metallic scales. It is pervious to the inward passageof gas and to the outward passage of reaction gases formed by the carbon in the base metal and oxygen. Likewise, the surface is free from a heavy continuous boundary of silica with the higher manganese contents abovenoted.

To indicate the improved results from our treatment, the following data are presented. This". I

data was obtained from sets of samples of silicon steel taken from the same coils of strip rolled 1 and finished in the same manner with the exception that the lime coating was applied, in one case, directly on the surface of the-metal, and in the other, on a layer of scaleiof the. type herein described. This hot-rolled strip had the following composition:

Per cent Carbon .047 Manganese .24 Phosphorus .014 Sulphur .019 Silicon 3.37

The hot-rolled strip was pickled in sulphuric acid solution until free from scale, dried and cold reduced from .08 inch to .026 inch gauge, con-. tinuously annealed by holding it at a temperature of 1675 F. for about 5 minutes in an atmosphere containing 18% hydrogen, balance substantially I nitrogen and having a moisture content corresponding to a dew point of 75 F. Part of the coils were pickled to remove the scale and the others left with the scale intact. Both sets of samples were then coated with lime and box to less than .03 inch gauge, continuously annealannealed at a temperature of 2000 F. in an atmosphere of nitrogen. After shearing, both sets were given a conventional stress-relief anheal for one hour at 1425" F. to relieve the shearing. stresses. samples to determine the core loss as expressed in watts per pound for both 10,000 and 15,000 gausses. Column A gives the data for the samples having the scale left thereon and column B the data for those with the scale removed.

Tests were then made on the carbon contents below 005% are readily obtainable inthe box annealing .eventhough the manganesebontent is substan'ti ally greater than .15

While we have shown and described one specific embodiment of our invention, it will be understood that this embodiment is merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims,

. We claim:

1. A method of treating silicon steel containing between 2.5 and 3.5% silicon comprising continuously annealingusaid steel in an atmosphere containing 18 to 23% hydrogen, balance substantially nitrogen and saturated with water vapor at room temperature, the temperature of said annealing being between 1600 and 1725 F., said atmosphere and temperature combining to de- Velop on the surface of said steel a well-defined pervious zone of low-silicon iron interspersed with silica 'and::iron silicates, and subsequently box annealing said steel with lime in contact with the surfaces thereof in the presence of said pervious zone between 1900 and 2100 F.'to further decarburize said steel and to develop the magnetic characteristics thereof.

2. A method of processing silicon steel strip, the steel thereof containingbetween .5 and 5% silicon comprising'hot rolling said strip to above .08 inch gauge', pickling said strip to remove the scale therefrom, cold reducing said pickled strip ing said pickled strip in an atmosphere comprising between 18 and 23% hydrogen, balance sub stantially nitrogen and saturated with water vapor at room temperature, the temperature of said annealing being between 1600 and 1725" 1 This decrease of watt loss which averages over 15 per cent, greatly contributes to the quality of sheets of the above class and .at the same time, the treatment does not impose any hardship or other burden on the regular processing operations.

7 From a theoretical standpoint, it is believed that the foregoing improvement is due to the annealing said strip with lime in contact with the surfaces thereof in the presence of said pervious zone at a temperature of approximately 2000" F. to further decarburize said steel and to develop controlled layer of scale and its behavior during 7 the box annealing. It is generally accepted that in order to obtain proper magnetic characteristics in which a high percentage of the grains are aligned so that the 110 plane and 100 direction are in therolling direction, it is necessary to reduce the carbon content well below 01%. It

has also been generally accepted that with manganese contents greater than .15%,.it is commercially impossible to lower the carbon to desired low degree and thereby permit the grains to grow in the preferred manner. However, by controlling the scale in themanner aforesaid,

the magnetic characteristics thereof.

CHARLES H. CAMPBELL. DONALD S. FERITO. CARL W. STOKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 886,668 -Wi1ks May 5, 1908 1,586,543 Westberg- June 1, 1926 2042,124 Reno, Jr. May 2, 1936 2,158,065 Cole et a1 May 16, 1939 2,287,467 Carpenteret a1. June 23, 1942 2,307,391 Cole etal. Jan. 5, 1943 

1. A METHOD OF TREATING SILICON STEEL CONTAINING BETWEEN 2.5 AND 3.5% SILICON COMPRISING CONTINUOUSLY ANNEALING SAID STEEL IN AN ATMOSPHERE CONTAINING 18 TO 23% HYDROGEN, BALANCE SUBSTANTIALLY NITROGEN AND SATURATED WITH WATER VAPOR AT ROOM TEMPERATURE, THE TEMPERATURE OF SAID ANNEALING BEING BETWEEN 1600* AND 1725*F., SAID ATMOSPHERE AND TEMPERATURE COMBINING TO DEVELOPE ON THE SURFACE OF SAID STEEL A WELL-DEFINED PERVIOUS ZONE OF LOW-SILICON IRON INTERSPERSED WITH SILICA AND IRON SILICATES, AND SUBSEQUENTLY BOX ANNEALING SAID STEEL WITH LIME IN CONTACT WITH THE SURFACES THEREOF IN THE PRESENCE OF SAID PERVIOUS ZONE BETWEEN 1900* AND 2100*F. TO FURTHER DECARBURIZE SAID STEEL AND TO DEVELOP THE MAGNETIC CHARACTERISTICS THEREOF. 